1. Field of the Invention
The present invention relates to a zoom lens system, and more specifically, to a zoom lens system, for use in a copying apparatus, where zooming is performed while a conjugate distance is kept constant.
2. Description of the Prior Art
In most of the copying apparatuses having a zooming function, zooming is still performed by a conjugate distance zooming method where a mirror is moved. However, if a zooming method is employed where a conjugate distance is kept constant by using a zoom lens system, continuous zooming can be performed only by moving the zoom lens system. As a result, the size of the copying apparatus can be reduced. For this reason, such zoom lens systems have been proposed to be incorporated into a copying apparatus recently.
For example, Japanese Laid-open Patent Application No. H1-123210 discloses a conjugate distance variable zoom lens system for use in a copying apparatus. The zoom lens system has five or eight lens elements with an f-number of 6.7 or 4, half view angles .omega. of 20.8.degree. to 16.1.degree. or 15.0.degree. to 12.5.degree. and magnifications of 0.5.times. to 2.0.times. or 0.64.times. to 1.54.times.. The zoom lens system is a three-unit lens system of a negative, positive, negative configuration, and zooming is performed by asymmetrically moving the first lens unit of a negative power and the third lens unit of a negative power. Although it is mentioned in the specification of this prior art that it offers no problem if there is a slight asymmetry in radius of curvature and axial thicknesses of the lens elements, the zoom lens system is, basically, symmetrical with respect to an aperture stop or a central lens element at a magnification of -1.times.. As advantages thereof the following are described. That is, aberrations can excellently be corrected in wide zoom ranges of -0.5.times. to -2.0.times. and 0.64.times. to 1.54.times.. Moreover, at a magnification larger than unity, a large distance can be secured between the lens system and an object surface, and at a magnification smaller than unity, a large distance can be secured between the lens system and an image surface.
U.S. Pat. No. 4,832,465 also discloses a conjugate distance variable zoom lens system for use in a copying apparatus. The zoom lens system has four and five lens elements grouped into two lens units with an f-number of 6.7, angles of view 2.omega. of 42.degree. to 36.degree. and magnifications of 1.42.times. to 0.64.times.. Zooming is performed by moving the entire lens system while varying the distance between the first lens unit of a positive power and the second lens unit of a negative power.
Among presently-known zoom lens systems for use in a copying apparatus, the ones which have the fewest number of lens elements are four-element lens systems. In such zoom lens systems, excellent aberration correction is achieved in various types in a zoom range of approximately -0.7.times. to 1.4.times.; however, when the magnification is outside the zoom range, the movement amount of the lens and variation in aberration are remarkably large. For this reason, it is impossible to put the zoom lens systems to practical use.